The proposed project will investigate the neuropharmacology of chlorinated alicyclic compounds, agents which enhance transmitter release at cholinergic synapses. The project will use a model in vitro system of the presynaptic nerve terminal, the synaptosome, which behaves much like an intact nerve with respect to transmitter release elicited by agents known to enhance acetylcholine release. The drugs proposed for study are chlorinated alicyclics which are simple molecules of small molecular weight and which are ideally suited for an investigation of structure-activity relationships. Electrophysiological studies will be conducted on the Mauthner-cell-motor fiber synapse (found in lower vertebrates) as a particularly suitable in situ system for the investigation of presynaptic alterations. Of the animals which possess Mauthner cells, the mosquitofish (Gambusia affinis) was selected for study because much toxicological information on chlorinated alicyclics is already available for this species. Synaptosomes from brain of mosquitofish will be used to investigate the mechanism of neurotoxic action of the chlorinated alicyclics by studying: a. acetylcholine release from synaptosomes under the influence of these drugs; and b. effects of these drugs on energy-dependent and -independent calcium fluxes in synaptosomes as the specific physiological parameter affected by these drugs. These results, along with synaptosomal binding of the drugs, will be correlated with electrophysiological effects of these drugs on intact nerves to elucidate structure-activity relationships.